GPS Global Positioning System

What is GPS?

The Global Positioning System (GPS) is a satellite-based navigation system
made up of a network of 24 satellites placed into orbit by the U.S. Department
of Defense. GPS works in any weather conditions, anywhere in the world, 24
hours a day. GPS is free of charge.

How it works

GPS satellites circle the earth twice a day in a very precise orbit and
transmit signal to earth. GPS receivers take this signal and use
triangulation to calculate the user's exact location. Essentially,
the GPS receiver compares the time a signal was transmitted by a
satellite with the time it was received. The time difference tells
the GPS receiver how far away the satellite is. Now, with distance
measurements from a few more satellites, the receiver can determine
the user's position and display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three
satellites to calculate a 2D position (latitude and longitude) and
track movement. With four or more satellites in view, the receiver
can determine the user's 3D position (latitude, longitude and
altitude). Once the user's position has been determined, the GPS
unit can calculate other information, such as speed, bearing, track,
trip distance, distance to destination, sunrise and sunset time
and more.

How accurate is GPS?

Modern 12 parallel channel receivers are quick to lock onto satellites
when first turned on and they maintain strong locks, even in dense
foliage or urban settings with tall buildings. Certain atmospheric
factors and other sources of error can affect the accuracy of GPS
receivers. Most GPS receivers are accurate to within 15 meters
on average.

Newer GPS receivers with WAAS (Wide Area Augmentation System) or
EGNOS capability can improve accuracy to less than three meters
on average.

Users can also get better accuracy with Differential GPS (DGPS),
which corrects GPS signals to within an average of three to
five meters. DGPS correction system consists of a network of
towers that receive GPS signals and transmit a corrected signal
by beacon transmitters. In order to get the corrected signal,
users must have a differential beacon receiver and beacon antenna
in addition to their GPS.

Clear vision to the sky is preferred

GPS does not work well inside buidings or vehicles. The signal is
lost or quality is lowered by obstacles. Antennas placed on car
roofs will give good view of all satellites, if not shadowed by
trees or buildings.